The present invention relates to a semiconductor device. The device in question is of a kind normally referred to as a tunneling device and particularly a resonant tunneling device.
The resonant tunneling device was originally described by L. L. Chang et al., Appl. Phys. Lett., 24, 595 (1974). The conventional resonant tunneling device is in the form of a diode, although the terminals are often called the `collector` and the `emitter`. Typically the device comprises a quantum well layer (e.g., GaAs) on either side of which is located a respective barrier layer (e.g., AlGaAs).
With this conventional kind of device, application of a bias voltage between the emitter and the collector causes conduction through the layers. At low voltages only a small current flows. However as the bias voltage increases, so does the current. When the energy matches that of a quasi-bound state in the quantum well, electrons can tunnel through the barrier layers so that current freely flows from emitter to collector. At this bias voltage, the quantum well is said to be in resonance, and this value of the bias may be termed the `resonant voltage`.
As the bias voltage is increased beyond the resonant voltage the energy becomes higher than that of the quasi bound state so that tunneling is inhibited. This gives rise to a region of negative differential resistance above the resonant peak in the IV characteristic. Thus sweeping the collector-emitter bias voltage from a voltage below the resonant voltage to a voltage above the resonant voltage shows a peak in the tunneling current centered around the resonant voltage, this peak in termed the tunneling peak.
Since tunneling is a very fast mechanism of charge transport, resonant tunneling devices offer the potential of extremely high speed operation. They have been described as oscillators (e.g., T. C. L. G Soliner et al., Appl. Phys. Lett., 45 1319 (1984)) and switches (e.g., S. K. Diamond et al., Appl. Phys. Lett., 54 153 (1989)). Oscillation frequencies up to 712 GHz have been reported. These devices can be fabricated as far infrared detectors.
Progress in this field has been reviewed in two parts by M. Henini et al., III-V's Review, 7 33 (1994) (Part 1) and III-V's Review, 746 (1994) (Part 2). This notes the possibilities offered by the fabrication of a three terminal transistor like device.
For production of a good RTD, optimization of the tunneling peak characteristics are required. A large difference between the magnitude of the tunneling current on resonance and off resonance, termed the peak to valley ratio, is required. A fast operating speed also requires a narrow tunneling peak, i.e., a small voltage range over which resonant tunneling occurs. In two terminal devices it is only possible to tune these characteristics by the fabrication of the structure, i.e., variation in the growth materials conditions etc. A three terminal device allows these properties to be tuned after the device is fabricated.